Yes the body language of calling Oswald over the passenger side window for a casual talk suggests they knew each other.
For what reason are you suggesting he called him over for a casual talk ?
You're still being elusive on where you think this all goes?
But ok, let's say they came across each other and we'll explore the possibilities;
1) They knew each other in not so friendly terms and Tippit pulled over to find out what Oswald was doing in the middle of the day on some back street and perhaps Tippit already had his gun unholstered and Oswald noticed and started moving away which prompted Tippit to get out?
2) They knew each and were friends, so why did Tippit get out of the car with an unholstered weapon?
3) Tippit didn't have a clue who the guy was but for some reason Oswald's actions, like perhaps changing directions, walking faster, looking away or whatever and this made Tippit suspicious, so he investigated?
4) Tippit was on the look out for a man 5 foot ten and slender so he pulled over and nonchalantly asked Oswald if he killed the President but this is absurd because as Lance points out, Tippit would pull over way behind and get out fully armed.
There are possibly other options but at this stage I pick what's behind box number 3!
Tippit was not near the Depository, this was about 45 minutes after the assassination, the weapon was a rifle, the chances that the assassin was walking around the back streets of Oak Cliff was very unlikely and Tippit doing his job saw a suspicious looking guy and just investigated.
What reinforces Oswald's suspicious behaviour is just shortly thereafter Oswald while outside of Brewers shoe shop was well aware of the Police and was actively avoiding them.
Mr. BELIN - When did he go in now? What did you hear at the time that he stepped into this lobby area?
Mr. BREWER - I heard the police cars coming up Jefferson, and he stepped in, and the police made a U-turn and went back down East Jefferson.JohnM